The present invention relates to a method for increasing the functionality of an epoxy resin.
Due to their desirable physical and chemical properties such as resistance to chemical attack and excellent strength, toughness and adhesion, epoxy resins are useful in a wide variety of commercial applications such as protective coatings, adhesives, civil engineering applications including floorings and mortars as well as structural applications and reinforced plastics.
Conventionally, the molecular weight of an epoxy resin is increased by the advancement reaction of a polyepoxide such as the diglycidylether of bisphenol A with a polyhydric phenol such as bisphenol A. For example, U.S. Pat. Nos. 3,006,891 and 3,006,892 described the advancement of a lower molecular weight polyepoxide such as the diglycidylether of a bisphenol with a dihydric phenol using lithium hydroxide or a lithium salt such as lithium chloride as the advancement catalyst. The resulting epoxy resins are essentially linear 1,2-epoxy resins exhibiting a functionality of 2 or slightly less than 2. More recently, advancement reactions have been catalyzed using a tertiary amine or phosphine such as benzyl dimethyl amine; quaternary ammonium or quaternary phosphonium compound such as benzyl trimethyl ammonium chloride and ethyl triphenyl phosphonium acetate. See, for example, U.S. Pat. Nos. 3,377,406 and 3,908,855.
In many applications, particularly those applications in which better high temperature performance and/or chemical resistance is needed, it is often desirable to employ an epoxy resin having an average epoxy functionality greater than 2 as the sole epoxy resin component or in combination with a more conventional difunctional epoxy resin. One method for increasing the epoxy functionality consists of adding a polyfunctional epoxy resin such as an epoxy novolac resin to the epoxy resin formulation. The epoxy novolac resin which is commonly the reaction product of epichlorohydrin with the reaction product of a phenol and formaldehyde is conventionally prepared at an average epoxy functionality of from 2.2 to 8 epoxy groups per molecule. However, these polyfunctional epoxy compounds are relatively expensive materials. In addition, the reactivities of the epoxy groups of the epoxy novolac resin or other polyfunctional epoxy resin and the epoxy groups of a more conventional difunctional epoxy resin such as the diglycidyl ether of bisphenol A are often different. These differences in reactivity of the epoxy groups can cause embrittlement or other problems upon curing a blend containing the two resin types. Moreover, in commercial application, due to the relatively high viscosities of the epoxy novolac resin, special processing equipment is often required.
Various phosphonium acid salt catalysts have been shown to increase the epoxy functionality, by branching, of an epoxy resin. However, these phosphonium acid catalysts must be employed in a relatively large amount to provide sufficient branching to significantly increase the epoxy functionality of the resin and the degree of reaction is difficult to control. See, for example, U.S. Pat. No. 4,352,918.
Alternatively, U.S. Pat. No. 4,251,594 teaches the preparation of an advanced epoxy resin having increased functionality resulting from branching reactions by reacting an epoxy resin which contains at least 500 ppm of an aliphatic halide with a dihydric phenol using an alkali metal hydroxide, perferably sodium or potassium hydroxide catalyst, in an amount of from 0.05 to 1 hydroxide equivalent per aliphatic halide equivalent in the reaction mixture. This aliphatic halide, e.g., chloride, is a residue formed during the preparation of the epoxy resin which is commonly prepared from epichlorohydrin and a bisphenol. The effective amounts of the hydroxides are stated to be dependent on the aliphatic halide content of the resin which unfortunately varies for different batches of the resin. Specifically, the reference teaches that the halide deactivates the alkali metal hydroxide catalyst by formation of the halide salt. This is a deficiency in the disclosed method.
In view of the deficiencies in the prior art methods for preparing epoxy resins having an average functionality of greater than two, it remains highly desirable to provide a method for preparing an epoxy resin having increased functionality which method does not exhibit the deficiencies of the prior art.